1. Field of the Invention
This invention relates to high frequency transmission line circuit structures, and in particular to such circuit structures having a chip circuit flip-mounted onto a slotline.
2. Related Art
Mounting flip-chips on a motherboard has been found to be an effective way to connect radio frequency circuit components together. The use of flip mounting provides an alternate attachment method that replaces the use of bond wires, backside metalization and vias, air bridges, and dielectric crossovers on the mother substrate. The conducting columns or bumps that connect the chip to the mother board can be formed using thermocompression bonding, solder, brazing material or adhesives.
Also, in such high frequency applications, the use of coplanar transmission lines is well established. Typical examples include conventional coplanar waveguides (ground-signal-ground lines), and slotlines. Slotlines may be formed by spaced opposite-polarity conductors, balanced ground-signal-signal-ground lines, and parallel-strip balanced lines. These coplanar transmission lines are particularly useful because of the simplified structure provided by having the signal-forming conductors on a single plane.
A slotline consists of two conductors mounted on one or both faces of a substrate. The conductors are spaced apart to form a slot. The slot is narrow enough for the conductors to be closely coupled, thereby making the conductor edges extending along the slot function as the primary transmission line. Slotlines are typically used to conduct balanced signals, such as are used in push-pull circuits. These circuits may or may not have a corresponding ground plane on the opposite or back side of the substrate. They may also be used for unbalanced signals, with one of the two conductors being at ground or other fixed common reference potential. Coplanar slotlines also have the advantages of not needing cross-over conductors and having fewer conductors compared to coplanar waveguides.
Slotlines are historically made as semi-infinite conductors, with each conductor covering all of the portion of the face of the substrate to the side of the slot. A slotline may also be formed by conductor strips having limited widths. Conventionally, a coplanar waveguide consists of two coupled slotlines having the same signal and opposite polarity.
As used herein, a slotline is distinguished from a coplanar waveguide primarily in that the transmission line formed by a slotline for a given signal consists of only two conductors. This is to say, each conductor is only associated with one other conductor to form the transmission line. On the other hand, a coplanar waveguide has three conductors, an inner signal conductor bounded by spaced outer conductors with the outer conductors having the same polarity and sharing in the conduction of current relative to the middle conductor. In a coplanar waveguide, the center conductor has a polarity opposite from the polarity of the two outer conductors and is associated with both outer conductors to form the transmission line.
This definition does not preclude the use of other conductors in association with one or both of two slotline conductors for transmitting other signals, for impedance matching or for other functions, so long as the other conductors have arbitrary signal polarities relative to those of the slotline conductors.
Also as used herein, a chip circuit is one or more electrical components formed in or on a chip substrate. Typically, integrated circuits are formed on a chip substrate to form a chip circuit. Other forms of circuit structure may also be formed on a chip. When the chip circuit is flip-mounted onto a base substrate with chip connection points adjacent to the substrate surface, the chip containing the chip circuit is referred to as a flip-chip. A flip-chip may contain one or more circuits, elements or devices, each of which may or may not be interconnected onto the chip.
In a common amplifier application, the chip circuit is an integrated circuit containing one or more transistors and other devices, such as resistors, capacitors and inductors. In a power chip having a plurality of transistors, the transistors may be individually or collectively connected to the motherboard. Impedance-matching for the composite power transistor is typically, although not always, accomplished on the mother board substrate in order to minimize the size of the chip substrate.